1. Field of the Invention
This invention relates to a communication terminal connected to a public network such as a telephone, a key telephone system, or a private branch exchange, and more particularly to a communication terminal having a function for receiving and displaying originator identification data to be transmitted from the public network to a destination.
2. Description of the Related Art
Analog telephone networks include a network having a function for notifying a destination subscriber of originator identification data, such as a telephone network for providing a service called "CLASS".
In an analog telephone network with such a function, originator identification data is notified to a destination in the following manner:
In the case of transmitting an incoming call signal by such an analog telephone network as has the "CLASS" service, a ringing signal of a predetermined frequency which continues two seconds is repeatedly transmitted at intervals of four seconds. Notification data for transmitting originator identification data is transmitted within an interval between the first and second ringing signals of the incoming call signal. A pose period of 0.5 seconds is inserted between the termination point of the first ringing signal and the start point of the notification data. The transmission period of the communication data is 575 ms. The format of the notification data is determined such that a synchronization signal of a "01"-repeated pattern, a mark signal, a message-type word, a data work count, a data section and a check sum are arranged in this order. The originator identification data contains "date/hour" and "originator number".
In the case of using a key telephone system as a communication terminal, the key telephone system detects, with the use of its key telephone control unit, notification data after detecting the first ringing signal. When the control unit has detected originator identification data from the notification data, it transmits the originator identification data to a destination key telephone, which in turn displays the data on its display unit. The user of the destination key telephone can recognize the originator of the call by seeing the originator identification data on the display.
The above-described conventional key telephone system, however, has the following drawback:
Suppose that the key telephone control unit has detected the first ringing signal transmitted from an analog network. Upon detecting the ringing signal, the key telephone control unit starts the transmission of a ringing signal to a destination key telephone, thereby causing the key telephone to ring so as to start notification of the incoming call. Subsequently, when the key telephone control unit has detected the originator identification data in the interval between the first and second ringing signals, it transmits the identification data to the destination key telephone, which in turn displays the data.
In other words, the key telephone rings before it displays the originator identification data.
Therefore, if the user performs a prompt off-hook operation immediately after he has heard the ringing, it is possible that he receives no originator identification data, since the transmission of the incoming call signal is ceased when a call response has been performed. A relatively large number of users perform a prompt off-hook operation in a conditioned reflex manner when they have heard a telephone ringing. Therefore, the above problem occurs in unexpectedly many cases.